We will continue to investigate descending inhibitory control of spinal nociceptive transmission. Our long-term objective is to clarify the organization and selectivity of centrifugal inhibitory influences on spinal dorsal horn neurons by quantitatively evaluating the effects of drugs and focal electrical stimulation applied in selected sites in the brain stem on both noxious and non-noxious spinal afferent input, correlating the spinal inhibition with the analgesia produced by the same treatments. The specific aims include: quantitative characterization and comparison of the inhibition of spinal dorsal horn neurons produced by opioids and/or focal electrical stimulation given in the brain stem; quantitative characterization of centrifugal inhibitory influences on visceral pain and comparison with inhibition of somatocutaneous pain; characterization of a pseudoaffective pressor reflex produced by noxious visceral stimulation and its centrifugal modulation; and investigation of the organization and interaction in the brain stem of descending inhibitory influences on spinal nociceptive transmission. These aims will be achieved by extracellular single unit recording in the lumbosacral spinal cord of the rat, intracerebral focal electrical stimulation and drug microinjections at the same sites in the brain stem, and intrathecal administration of drugs. These experiments will initiate important investigations into visceral pain, characterizing the stimulus and its relationship to a pseduoaffective pressor reflex, neurons responding in the spinal dorsal horn, somatovisceral convergence, and descending modulation. In all experiments, use of noxious and non-noxious stimuli will be employed to address whether a descending system selectively affects noxious afferent input. Descending inhibitory influences of drugs and stimulation will also be compared and contrasted with their analgetic effects behaviorally. Results of these experiments will provide new information about centrifugal modulation of somatic and visceral pain and clarify the organization in the brain stem of descending systems of inhibition/analgesia.